Dunno really.
Can depend who the sample is - I don't know how polling companies try and make their polls representative across the whole population. Presume for example that any poll carried out among the readers of a particular newspaper don't qualify.
And I think there's still quite a lot of 'not really sure' even if that doesn't register in polls as 'don't know' - personally, I'm still not entirely sure if I will bother to vote or if so who for.
It also depends whether you ask a question who you'd vote for (in general), or who you intend to vote for in your constituency (and then there's who they actually will vote for on the day) - in the latter, some people will vote tactically / reluctantly on the day to get their 'least worst' credible option / to 'stop the other lot getting in', but will express a preference in general, knowing that party is unlikely to win in their constituency.
And in opinion polls, there's a tendency for people to say they will vote for party A, when they usually support party B but aren't happy with what they are up to at the moment - but when it comes to a real vote, they are less likely to vote for party A.